Gene therapy provides a method for transferring a desired gene to a subject with the subsequent in vivo expression thereof. Gene transfer is generally accomplished by genetically modifying the subject's cells or tissues ex vivo, using an appropriate vector, and reintroducing the modified cells into the host. Alternatively, genetic material can be transferred directly into the cells and tissues of the subject.
A number of viral based systems have been used for gene delivery. For example, retroviral systems are known and generally employ packaging lines which have an integrated defective provirus (the "helper") that expresses all of the genes of the virus but cannot package its own genome due to a deletion of the packaging signal, known as the psi (.psi.) sequence. Thus, the cell line produces empty viral shells. Producer lines can be derived from the packaging lines which, in addition to the helper, contain a viral vector which includes sequences required in cis for replication and packaging of the virus, known as the long terminal repeats (LTRs). The gene of interest can be inserted in the vector and packaged in the viral shells synthesized by the retroviral helper. The recombinant virus can then be isolated and delivered to a subject. (See, e.g., U.S. Pat. No. 5,219,740.)
A critical factor in achieving effective gene transfer is the ability to obtain viral infection of a sufficient proportion of the contacted cells. Often in gene transfers, less than one-third of the cells contacted by a virus ex vivo are effectively modified. Furthermore, large numbers of genetically modified cells are required for most gene delivery applications. Thus, where the efficiency of viral infection is low, the difficulty in obtaining a sufficient number of genetically modified cells can present a limiting step in achieving effective therapy. There thus exists a need for efficient and effective genetic modification of mammalian cells. The present invention satisfies this need and provides related advantages as well.